An Australian being held as a terror suspect at Guantánamo Bay today won the right to be registered as a British citizen.

The decision means David Hicks will be able to call on British authorities to help secure his release.

A judge at the high court in London ruled that the home secretary, Charles Clarke, had "no power in law" to deprive Mr Hicks of British citizenship and said he "must be registered".

Mr Hicks is originally from Adelaide, southern Australia. The 30-year-old's mother was born in the UK and emigrated to Australia as a child - the basis of his case for securing a British passport.

He was arrested by US forces in Afghanistan in December 2001 when he was allegedly fighting for the Taliban, and it has been claimed he attended terrorist training camps in Afghanistan and Pakistan.

He faces charges including conspiracy to commit war crimes and aiding and abetting the enemy, all of which he denies.

Mr Hicks's lawyers argue that the Australian government has refused to plead for his release and has made no attempt to prevent his trial by a US military commission.

They say that, in contrast, the UK government has acted to remove all nine of its citizens imprisoned at Guantánamo, and would be compelled to call for the release of Mr Hicks.

After today's ruling, his lawyers will now press the British government to make arrangements for him to take the required citizenship oath and pledge.

They will then urge the Home Office to seek his release from the detention centre, in Cuba, and bring him to the UK.

At the high court this morning, Mr Justice Collins gave the home secretary permission to appeal against his judgment on citizenship, but refused to suspend his decision pending any appeal.

At a hearing last month, lawyers for Mr Hicks argued that Mr Clarke's approach to their client was "wholly inconsistent with the way in which he has dealt with all of the other British citizens detained in Guantánamo Bay" because he was not trying to help him.

Amnesty International has urged the government to help a number of former UK residents, who are not British citizens, being held at Guantánamo.

Amnesty says there are at least six UK residents among around 200 detainees who have been hunger striking against their continued detention without trial.